FIGS. 1 and 2 show the free end of a known type of turbine rotor blade 1. At its free end 3, the blade presents an open cavity or bathtub 2 defined by an end wall 8 and a rim 9 of closed outline that projects from the periphery of said end wall. The rim runs along the suction side 4, the leading edge 6, the pressure side 5, and the trailing edge 7 of the blade.
When mounted inside a turbojet, the rotor blades 1 are surrounded by a ring 30, shown in FIG. 2. The rim 9 serves to prevent the end wall 8 from rubbing against the ring 30. In addition, it serves to optimize the clearance J between the free end 3 of the blade and the ring 30 so as to limit gas flow in this region.
The aerodynamic design of a turbine blade requires the distance from the leading edge to the trailing edge of the blade to be longer on the suction side than on the pressure side.
The gas passing over the suction side of the blade has a longer distance to travel than the gas passing over the pressure side, so it has a comparatively greater flow speed. Thus, on the suction side, the pressure exerted by the gas on the blade is lower than that exerted on the pressure side. This establishes a pressure gradient that enables the rotor to be turned.
The pressure gradient is also at the origin of a certain volume of gas F passing through the clearance J between the rim 9 and the ring 30, this gas F going from the pressure side towards the suction side.
Unfortunately, this phenomenon is harmful to the efficiency of the turbine since the gas F does not contribute to rotating the rotor.